Production method for semiconductor device

ABSTRACT

A method of manufacturing a semiconductor device that provides a semiconductor device having improved channel mobility includes a process of forming a gate insulation film of silicon oxide film, silicon nitride film or silicon oxide nitride film or the like on a silicon oxide substrate, and following formation of the gate insulation film on the silicon oxide substrate with heat treatment for a given time at a temperature range of 900° C. to 1000° C. in an atmosphere containing not less than 25% H 2 O (water).

TECHNICAL FIELD

The present invention relates to a method of manufacturing a field-effect transistor, a bipolar transistor, a thyristor or other such semiconductor device having a metal-insulation film (or oxide film)-semiconductor structure that uses a silicon carbide substrate.

BACKGROUND ART

Silicon carbide is a semiconductor material having excellent characteristics, such as a large band gap, high thermal conductivity, high electron saturation drift velocity and high dielectric breakdown voltage, and is drawing attention as a material for next-generation low-loss power device elements. As in the case of silicon semiconductors, thermal oxidation can be used to form an oxide film from silicon carbide, making a field-effect transistor having a metal-oxide film-semiconductor structure (MOSFET) a leading candidate for silicon carbide power device elements. But with silicon carbide substrate MOSFETs manufactured with current technology, channel mobility is far lower than the mobility predicted from the electron mobility of bulk silicon carbide. This is also the case with a MIS-type FET that uses an oxide film/nitride film/oxide film or the like as the insulation film. Considerable research has been conducted aimed at improving the channel mobility, and there are a number of proposals relating to a method of forming the oxide film/silicon carbide interface.

For example, in thermal oxidation of pyrogenic oxidation in which water is produced by the combustion of hydrogen and oxygen, JP-A HEI 11-31691 discloses a method in which the flow rate of hydrogen is increased to more than the flow ratio of 1:1 between hydrogen and oxygen. The embodiments thereof describe using this method to form a gate oxide film, decreasing the interface state density in a MOS capacitor using P-type silicon carbide.

With respect to the method of oxidizing a silicon carbide substrate, U.S. Pat. No. 5,972,801 discloses using the thermal oxidation method to form a gate oxide film in a range of 1050° C. to 1100° C., and following this by heat treatment at 600° C. to 1000° C. in an atmosphere containing H₂O. In the embodiments thereof, oxygen is passed through pure water heated to 95° C. (subjected to bubbling) to produce an atmosphere containing H₂O.

Because a conventional MIS- or MOS-type FET that uses a silicon carbide substrate has a channel mobility that is much lower than the electron mobility of bulk silicon carbide, the intrinsic properties of silicon carbide are not reflected in the device characteristics. That is, the FET On resistance (R_(on)) is much higher than the value that is theoretically predicted based on the property values of silicon carbide. Although silicon carbide having the crystalline structure known as 4H (4H-SiC) has a bulk electron mobility in the order of 900 cm²/Vs, the channel mobility of a MOSFET formed by the usual thermal oxidation method is a very low 5 to 10 cm²/Vs. To improve this, Reference 1 (K. Ueno et al., Mat. Sci. and Eng. B61-62 (1999) 472-474) relates to an improvement by using surface treatment prior to formation of the gate oxide film; Reference 2 (J. A. Cooper et al., Mat. Res. Soc. Proc. Vol. 572, pp 3-14) relates to an improvement based on the use of a low activation annealing temperature; and Reference 3 (G. Y. Chung et al., IEEE Electron Device utt. 22, 176 (2001)) relates to re-oxidation treatment in an NO (nitrous oxide) atmosphere. These references report channel mobilities in the order of 20 to 35 cm²/Vs, so in view of the bulk electron mobility, dearly, there still is room for improvement.

The present invention was proposed to resolve the above problem, and has as its object to provide a method of manufacturing MIS- and MOS-type semiconductor devices with high channel mobility that use a silicon carbide substrate.

DISCLOSURE OF THE INVENTION

The present invention provides a method of manufacturing a semiconductor device that includes a process of forming a gate insulation film on a silicon carbide substrate, further comprising a process of heat treatment for a given time at a temperature of 900° C. to 1000° C. in an atmosphere containing at least 25% H₂O (water), after formation of the gate insulation film on the silicon carbide substrate.

The given heat treatment time in the atmosphere containing H₂O is from 1 to 5 hours.

The gate insulation film is silicon oxide film, silicon nitride Mm or silicon oxide nitride film, or a multilayer film that includes one of these films.

The gate insulation film is a film formed by a thermal oxidation method or a deposition method, or by a method that is a combination of those methods.

The thermal oxidation method uses an atmosphere of dry oxygen.

The atmosphere containing H₂O (water) further contains one or both of O₂ (oxygen) gas and an inert gas.

The H₂O (water) contained in the atmosphere containing H₂O (water) is produced by thermal reaction of H₂ (hydrogen) gas and O₂ (oxygen) gas.

The method of manufacturing a semiconductor device according to the present invention further comprises a group of processes that are conducted after the heat treatment process in the atmosphere containing H₂O and that adopt a maximum temperature of 800° C.

The silicon carbide substrate has a (000-1) plane orientation.

The channel mobility of the semiconductor device thus manufactured can be substantially improved by heat treatment at a temperature of 900° C. to 1000° C. in an atmosphere containing H₂O after the gate insulation film has been formed on the silicon carbide semiconductor substrate, as described above.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a flow chart showing the process of manufacturing a MOSFET according to the present invention.

FIG. 2 is a diagram showing the gate voltage dependency of the field-effect mobility of a MOSFET manufactured by the method of the present invention.

FIG. 3 is a diagram showing the heat-treatment moisture concentration dependency of the maximum field-effect channel mobility of a MOSFET using a 4H-SiC (0001) substrate manufactured by the method of the present invention.

FIG. 4 is a diagram showing the heat-treatment moisture concentration dependency of the maximum field-effect channel mobility of a MOSFET using a 6H-SiC (0001) substrate manufactured by the method of the present invention.

FIG. 5 is a cross-sectional diagram of a MOS-type semiconductor device according to an embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

In a method of manufacturing a semiconductor device that includes a process of forming a gate insulation film on a silicon carbide substrate, this invention comprises following the process of forming the gate insulation film on the silicon carbide substrate with heat treatment in an atmosphere containing H₂O in order to improve the channel mobility of the manufactured FET.

The above heat treatment in an atmosphere containing H₂O is carried out after formation of the gate insulation film. However, the effect of the heat treatment also depends on gate insulation film formation conditions (temperature, atmosphere and so forth), and if the conditions are different, there may be cases in which no improvement in channel mobility is seen. Also, in the heat treatment of the present invention, in addition to the moisture concentration of the atmosphere containing H₂O, there are clear ranges at which conditions relating to heat treatment temperature and heat treatment time are optimal, and improvement in channel mobility may not be seen in cases where there is deviation from these conditions. This behavior is different from that discussed with reference to interface state density in the prior art.

In the following, details of an aspect of an embodiment of the invention are described, with reference to the flow chart of FIG. 1 showing the procedure of manufacturing a MOS-type FET.

FIG. 5 shows an example of a MOS-type FET using a silicon carbide substrate, manufactured in accordance with the invention. With reference to the drawing, a source region 2 and a drain region 3 are provided in a silicon carbide substrate 1 as sp aced by a channel region from each other. A gate insulation film 5 is provided on the channel region, and a gate electrode 8 is provided on the gate insulation film 5.

The source region 2 is electrically connected to a source electrode 6 and the drain region 3 is electrically connected to a drain electrode 7, and the source electrode 6, drain electrode 7 and gate electrode are each insulated by an insulation film 4.

The MOS-type FET shown in FIG. 5 was actually fabricated and the mutual conductance (G_(m))-gate voltage (V_(g)) properties used to investigate the dependency of the field-effect channel mobility (μ_(FE)) in the linear region (drain voltage V_(d)=0.1 V) on the gate insulation film formation method and subsequent beat treatment process. As the silicon carbide substrate 1, there were used two types of substrate: a P-type 4H-SiC (0001) epitaxial substrate (having a difference of approximately 5×10¹⁵ cm⁻³ between acceptor density Na and donor density), and a P-type 6H-SiC (0001) epitaxial substrate (having a difference of approximately 5×10¹⁵ cm⁻³ between acceptor density Na and donor density). During fabrication of all samples, temperatures were raised and lowered at ±5° C./minute. Specific details of the processes of manufacturing the MOS-type FETs will now be described, in the order shown in FIG. 1.

-   I). Alignment mark formation process:     -   I-1. The silicon carbide substrate is cleaned using RCA         cleaning.     -   I-2. Photolithography is carried out to form alignment marks         used for photolithography.     -   I-3. RIE (reactive ion etching) is used to form alignment marks. -   II). Source/drain region formation process:     -   II-1. Deposition is used to form an oxide film for an ion         implantation mask for the source/drain regions.     -   II-2. After source/drain region photolithography, dry etching or         BHF (buffered hydrofluoric acid) is used to etch the deposited         oxide film and expose the ion-implanted source/drain regions.     -   II-3. Nitrogen, phosphorus or arsenic ions are implanted at a         substrate temperature of 500° C. to 1200° C. to form the         source/drain regions. In this example, phosphorus ions were         implanted at a substrate temperature of 500° C. -   III). Ion implantation region activation annealing process:

III-1. In an inert gas atmosphere, activation annealing is performed at a temperature range of 1100° C. to 2000° C. In this example, activation annealing was performed at 1500° C. for 5 minutes.

-   IV). Process of forming insulation film between layers:     -   IV-1. Inter-layer oxide insulation film is formed by the         deposition method.     -   IV-2. After photolithography of the element formation region         (active region), dry etching or BHF is used to etch the         deposited oxide film and expose the active region. -   V). Gate insulation film formation process and heat treatment     process:     -   V-1. The substrate is cleaned using RCA cleaning.     -   V-2. (Sacrifice) oxide film is formed on the substrate by         thermal oxidation, and diluted hydrofluoric acid is used to         remove the oxide film (sacrificial oxide process).     -   V-3. Gate insulation film is formed at 1100° C. to 1250° C.         using the thermal oxidation method or deposition method, or a         combination of both. In the example, as the gate insulation         film, a silicon oxide film approximately 42 nm thick was formed         by thermal oxidation at 1200° C. in a dry oxygen atmosphere.     -   Silicon oxide film is mentioned as a typical gate insulation         film. However, since an extremely thin silicon oxide film is         known to exist in the interface between the semiconductor         substrate and the gate insulation film, a silicon nitride film         or silicon oxide nitride film may also be used. Also, using a         multilayer film that includes silicon oxide film, silicon         nitride film or silicon oxide nitride film has the effect of         increasing the long-term reliability of the gate insulation         film.     -   V-4. After the gate insulation film has been formed, if required         it can be annealed for 30 minutes or more at 1100° C. to         1300° C. in an inert gas. In the example, annealing was         performed in argon for 30 minutes at 1200° C.     -   V-5. Heat treatment in an atmosphere containing at least 25% H₂O         was performed at a temperature range of 900° C. to 1000° C. for         between 1 hour and 5 hours. In the example, it was performed at         950° C. for 3 hours.     -   There are a number of methods of creating an atmosphere         containing H₂O. In this example, the atmosphere containing H₂O         was produced by the thermal reaction (the pyrogenic method) of         H₂ (hydrogen) and O₂ (oxygen), which facilitates controlling the         H₂O concentration. By including oxygen or inert gas in the         atmosphere, the moisture concentration is readily controlled. In         the example, Ar gas was introduced. The [H₂]/[O₂] ratio between         Ha gas flow rate [H₂] and O₂ gas flow rate [O₂] was varied         within the range 0 to ⅔, which corresponds to a moisture         concentration of 0% to 50%. -   VI). Gate electrode formation process:     -   VI-1. The gate electrode can be a metal film, such as aluminum         (Al) or molybdenum (Mo); a silicide film, such as W         (tungsten)-Si₂ film, Mo—Si₂ film or Ti (titanium)-Si film; or an         N-type or P-type silicon gate electrode. The temperature at         which a metal gate electrode is formed on the substrate, while         depending on the metal used, is not higher than 500° C. In the         case of a silicon gate, the temperature is 500° C. to 800° C. In         the example, an Al electrode was formed through the resistance         heating method.     -   VI-2. After gate electrode photolithography, the gate electrode         is formed through etching of the electrode formed in VI-1. -   VII). Contact Hole and Contact Electrode Formation Process:     -   VII-1. Dry etching or wet etching is used to form contact holes         in the insulation film over the source/drain regions.     -   VII-2. Immediately after forming the contact holes, a film         composed of a metal, such as Ni, Ti or Al, or of laminated         layers thereof, is formed by vapor deposition and         photolithography is used to form contact electrodes. Also, if         necessary, annealing at around 800° C. is performed in an inert         gas atmosphere. -   VIII). Metal wiring formation process:     -   VIII-1. A film of a metal, such as Ni, Ti or Al, or of a         lamination thereof, is formed. In the example, it was formed of         Al using resistance heating.     -   VIII-2. Metal wires are formed by dry etching or wet etching the         above laminated film. In this example, after deposition of Al         using resistance heating, wet etching was used.     -   VIII-3. If required, the formed metal wiring is heat-treated in         an inert gas atmosphere. -   IX). Protective film formation process:     -   IX-1. After completing the formation of contact electrodes and         metal wiring, a low-temperature CVD process is used to form a         protective film of silicon oxide thereon at a temperature not         higher than 400° C.     -   The above is the procedure used when self-alignment is not used.         If self-alignment is used, the procedure will comprise alignment         mark formation process I, process II of forming insulation film         between layers, gate insulation film formation process and heat         treatment process III, gate electrode formation process IV,         source/drain region formation process V, ion implantation region         activation annealing process VI, contact hole and contact         electrode formation process VII, metal wiring formation process         VIII, and protective film formation process IX.

Typical respective process conditions pertaining to the following processes are shown below, which processes are gate insulation layer formation 1, annealing 2 in inert gas and heat treatment 3 in H₂O-containing atmosphere.

-   1. Gate insulation layer formation: 140 minutes at 1200° C. in dry     oxygen atmosphere, -   2. Post-annealing in inert gas: 30 minutes at 1200° C. in argon gas,     and -   3. Heat treatment in H₂O-containing atmosphere: Pyrogenic oxidation     conditions are [H₂]/[O₂]=0 to ⅔, substrate temperature of 950° C.     and heat treatment time of 180 minutes.

In FIG. 2 is a diagram showing the gate voltage dependency of the field-effect channel mobility of a MOSFET manufactured by the method of above-described example.

In FIG. 2, curve (a) is the channel mobility of a MOSFET (MOSFET (a)) fabricated using a 4H-SiC (0001) substrate, and curve (b) is the channel mobility of a MOSFET (MOSFET (b)) fabricated using a 6H-SiC (0001) substrate.

In each case, the MOSFET heat treatment conditions, as described in V-4, were 950° C. for 3 hours, in an atmosphere with a moisture concentration of 50% (H₂O₂=0 to ⅔).

As can be clearly seen from FIG. 2, the channel mobility of MOSFET (a) was approximately 50 cm²/Vs and the channel mobility of MOSFET (b) was approximately 95 cm²/Vs.

Also, as can be seen from FIG. 2, in the MOSFETs, whichever the substrate used, the decrease from the maximum channel mobility value μ_(FEmax) to the high gate voltage side (25 V) was small, being in the order of 20%. The maximum value μ_(FEmax) in the case of a gate insulation film formed by the usual thermal oxidation method is 5 to 10 cm²/Vs in the case of a MOSFET with a 4H-SiC (0001) substrate, and 35 cm²/VS in the case of a MOSFET with a 6H-SiC (0001) substrate, from which it can be seen that with both MOSFETs (a) and (b) manufactured by the method of this invention, major increases in channel mobility μ_(FE) were achieved. These increases in the channel mobility μ_(FE) were also confirmed in MOSFETs using 4H-SiC (000-1) and 6H-SiC (000-1) substrates. In particular, it was confirmed that the use of a (000-1) substrate improved channel mobility over a wide temperature range from 800° C. to 1100° C.

FIG. 3 is a diagram showing the heat-treatment moisture concentration dependency of the maximum field-effect channel mobility of a MOSFET manufactured using a 4H-SiC (0001) substrate. In FIG. 3, the solid bullets (●) are plot points representing the maximum MOSFET channel mobility value μ_(FEmax) at a moisture concentration in the heat-treatment atmosphere of 0% (corresponding to annealing in a dry oxygen atmosphere), 13%, 25%, and 50%. As is clear from the figure, the maximum μ_(FEmax) value of the fabricated MOSFETs increases with the increase in the moisture concentration, with the increase in the maximum μ_(FEmax) value being particularly pronounced in the case of MOSFETs fabricated in an atmosphere having a moisture concentration of 25% or more.

For reference, the channel mobilities of MOSFETs fabricated in the above process using heat treatment for 180 minutes at 850° C. (Δ) or 1050° C. (∇) in an atmosphere containing 25% H₂O, and MOSFETs thus fabricated using heat treatment for 60 minutes at 950° C. in an atmosphere containing 25% H₂O (□) are shown. In these FETs, the maximum channel mobility was low, being in the order of 8 cm²/Vs.

Such maximum channel mobility is substantially the same as that obtained using a gate insulation film formed using just the ordinary thermal oxidation.

In FIG. 4, the solid bullets (●) are plot points representing the maximum channel mobility value μ_(FEmax) of a MOSFET using a 6H-SiC (0001) substrate fabricated at a moisture concentration in the heat-treatment atmosphere of 25% and 50%, The solid squares (▪) in the figure denote the values of a MOSFET not subjected to heat treatment in an H₂O-containing atmosphere after formation of the gate insulation film by thermal oxidation. From the figure, it can be seen that even in the case of a MOSFET that uses a 6H-SiC (0001) substrate, compared to the maximum channel mobility of a MOSFET that uses a gate oxide film formed by ordinary thermal oxidation, an increase in the moisture concentration of the atmosphere is accompanied by a major increase in mobility.

In particular, the channel mobility was approximately 97 cm²/Vs in the case of a MOSFET manufactured using an atmosphere having a moisture concentration of 50%.

In the method of manufacturing a semiconductor device according to the present invention, the processes after the heat treatment in an H₂O-containing atmosphere does not include a process at or above a temperature of 800° C., and as a result, fabrication of the semiconductor device can be completed in a state in which the increase in channel mobility resulting from the heat treatment in an H₂O-containing atmosphere is maintained. When the processes following the heat treatment in the H₂O-containing atmosphere included a process at or above the heat treatment temperature, it was confirmed that the effect was reduced. To take one example, in the case of a MOSFET using a 6H-SiC (0001) substrate that was subjected to heat treatment at 1000° C. for 2 minutes after the heat treatment in an H₂O-containing atmosphere, channel mobility decreased from 95 cm²/Vs to around 70 cm²Vs. However, the effect is not completely extinguished.

The above embodiment was described with reference to a basic transverse type MOS-type FET. However, the method of manufacturing a semiconductor device in accordance with this invention can also be applied with the same effect to other semiconductor devices, such as a vertical type MOS- (MIS-)type FET, an insulated gate bipolar transistor (IGBT) and a MOS-type thyristor.

INDUSTRIAL APPLICABILITY

As described in the foregoing, in a process of manufacturing a MIS- or MOS-type FET using a silicon carbide substrate, in accordance with this invention, a MIS- or MOS-type semiconductor device having high channel mobility can be realized by, after forming the gate insulation film, using heat treatment for a given time in a water-containing atmosphere.

Also, channel mobility can be improved by using heat treatment at a temperature of 900° C. to 1000° C. in an atmosphere containing H₂O (water) after forming a gate insulation film composed of a single or multiple layers of silicon oxide film, silicon nitride film or silicon oxide nitride film used in a transistor or the like on a silicon carbide region. 

1. A method for fabricating a semiconductor device comprising: forming a gate insulation film on a silicon carbide substrate; and heat-treating the gate insulation film for 1 to 5 hours at a temperature of 900° C. to 1000° C. in an atmosphere containing at least 25% water.
 2. The method of manufacturing a semiconductor device according to claim 1, wherein the gate insulation film is silicon oxide film, silicon nitride film or silicon oxide nitride film.
 3. The method of manufacturing a semiconductor device according to claim 1, wherein the gate insulation film is a multilayer film that includes silicon oxide film, silicon nitride film or silicon oxide nitride film.
 4. The method of manufacturing a semiconductor device according to claim 1, wherein the gate insulation film is a film formed by a thermal oxidation method or a deposition method, or by a method that is a combination of those methods.
 5. The method of manufacturing a semiconductor device according to claim 4, wherein the thermal oxidation method uses an atmosphere of dry oxygen.
 6. The method of manufacturing a semiconductor device according to claim 1, wherein the atmosphere containing water further contains one or both of O₂ (oxygen) gas and an inert gas.
 7. The method of manufacturing a semiconductor device according to claim 1, wherein the water contained in the atmosphere containing water is produced by thermal reaction of H₂ (hydrogen) gas and O₂ (oxygen) gas.
 8. The method of manufacturing a semiconductor device according to claim 1, further comprising a group of processes that are conducted after the heat treatment process in the atmosphere containing water and that adopt a maximum temperature of 800° C.
 9. The method of manufacturing a semiconductor device according to claim 1, wherein the silicon oxide substrate is formed thereon with a silicon oxide region having a (000-1) plane orientation. 